Many aircraft are powered by jet engines. In most instances, jet engines include one or more gas-powered turbine engines, auxiliary power units (APUs), and/or environmental control systems (ECSs), which can generate both thrust to propel the aircraft and electrical and pneumatic energy to power systems installed in the aircraft. Although most aircraft engines are generally safe, reliable, and efficient, the engines do exhibit certain drawbacks. For example, the turbine engines can be sources of unwanted noise, especially during aircraft take-off and landing operations. Moreover, APUs and ECSs can be sources of unwanted ramp noise while an aircraft is parked at the airport. Thus, various governmental and aircraft manufacturer rules and regulations aimed at mitigating such noise sources have been enacted.
To address, and at least somewhat alleviate, the unwanted noise emanating from aircraft noise sources, and to thereby comply with the above-noted rules and regulations, various types of noise reduction methods have been developed. For example, one noise reduction method that has been developed for use in aircraft ducts is a noise suppression panel. In many instances, noise suppression panels are flat or contoured, and include either a bulk noise suppression material or a honeycomb structure disposed between a backing plate and a face sheet. The noise suppression panels are typically placed on the interior surface of an engine or APU inlet and/or outlet ducts, as necessary, to reduce noise emanations.
Although the above-described noise suppression panels do exhibit fairly good noise suppression characteristics, the panels also exhibit certain drawbacks. For example, the bulk noise suppression material can be costly to manufacture and may not provide sufficient resistance to fluid wicking. The honeycomb structure may be difficult to conform to contoured surfaces. Additionally, the honeycomb structure can also be difficult to bond to the backing plate and/or face sheet. Moreover, when the honeycomb structure is combined with an inexpensive perforate face sheet it provides attenuation over only a relatively narrow frequency range. Currently, more expensive facesheet materials are capable of effective performance over a wide range of frequencies.
Hence, there is a need for a noise suppression material that is less costly to manufacture as compared to known materials, and/or can be readily conformed to contoured surfaces, and/or can be readily bonded to backing and/or face sheets, and/or is effective over a relatively wide frequency range. The present invention addresses one or more of these needs.